The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air hearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, HDDs have been desired to store more information in its limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, it is useful to increase the quality of the magnetization transition which determines the signal-to-noise ratio (SNR) of the bit information stored to the magnetic medium. In order to achieve this with conventional techniques, a write bubble that is faster than the transition speed of the recording medium is generated. Therefore, many attempts have been made to shorten a magnetic circuit length for a magnetic head in order to further improve the high frequency properties.
Microwave-assisted magnetic recording (MAMR) has been researched for use as a recording method for improving surface magnetic recording density of magnetic media. In MAMR, a magnetic field is exerted by a main pole which applies an alternating current (AC) field from a spin-torque oscillator (STO) to a medium. Applying an AC field to a medium reduces the coercivity of the medium, which facilitates high-quality recording. Therefore, for MAMR to be more efficient, an STO which generates a sufficiently large AC field should be developed. With a STO structure as described in IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008 or J. Appl. Phys. 109, 07B741 (2011), and as shown in FIG. 1 according to the prior art, the STO 100 comprises a FGL 102 for generating an AC field, a spacer 104, and a spin polarization layer (SPL) 106 for transmitting spin-polarized torque. The SPL comprises a material having strong vertical anisotropy energy. The STO is also charged by a current from the SPL toward the FGL. During this charging, a spin torque oriented in the same direction as the magnetization of the FGL acts on the magnetization of the SPL, and a spin torque oriented in an antiparallel direction to the magnetization of the SPL acts on the magnetization of the FGL. The STO also comprises a cap layer 112 positioned above the FGL 102 and an underlayer positioned below the SPL 106. The STO is positioned between the main pole 108 and the trailing shield 110.
Spin torques are described in detail in J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1, 1996. Because a perpendicular field is applied to the STO, the magnetization of the SPL is stable vertically. The magnetization of the FGL, however, oscillates while having a large in-plane component. Oscillation of the STO in this structure is called T-mode oscillation because the SPL and the FGL oscillate in a T-shape.
A different STO structure is described in Japanese Unexamined Patent Publication No. 2013-65385, Application No. 2011-204843. With this structure, as shown in FIG. 2 according to the prior art, the STO 200 comprises a FGL 102 for generating an AC field, a spacer 104, and a SPL 106 for transmitting a spin-polarized torque. The differences between the STO 100 in FIG. 1 and STO 200 in FIG. 2 are that the magnetization of the SPL 106 is effectively oriented in the in-plane direction of the film, and both the FGL 102 and the SPL 106 in STO 200 oscillate. Specifically, a current is charged from the FGL toward the SPL, and a structure is used in which the SPL has a thin film thickness and a low vertical anisotropy field such that the anisotropy field of the SPL is effectively zero. This structure inverts quickly because inversion of the magnetization of the SPL is not delayed by switching the polarity of the write head field, which is useful in high-speed transfer recording. Oscillation of the STO 200 is called AF-mode oscillation because the SPL and the FGL oscillate while maintaining an antiparallel state.
A feature demanded of an STO is improvement in the generated AC field, which may be accomplished by increasing the spin torque acting on the FGL. Since the size of the spin torque is inversely proportional to the density of the current to the STO, increasing the application current obtains a higher AC field strength. FIG. 3 shows a relationship between AC field strength and application current in an STO structure using CoFe, while FIG. 4 shows a state of magnetization of the FGL and the SPL, as viewed from a side opposite the medium, when the charging current is 4 mA, 8 mA, and 18 mA.
The state of magnetization is a result of a micromagnetic simulation numerical calculation. FIG. 3 reveals that increasing the current charging the STO increases AC field strength, but too high a charging current attenuates AC field strength. As suggested by the state of FGL magnetization in FIG. 4, a major contributor to increasing AC field strength is a large in-plane component of FGL magnetization near the spacer boundary with the FGL. At greater distances from the spacer boundary with the FGL, however, FGL magnetization is greater in a direction perpendicular to the surface of the film, which contributes little to improving AC field strength. This is because the spin torque acting on the FGL is strongest near the spacer boundary, and becomes weaker as the distance from the spacer boundary increases. Because applying more charging current to the STO produces too high a spin torque near the spacer boundary with the FGL, FGL magnetization is disordered and develops multi domains, while there is little increase in the film surface component of FGL magnetization at greater distances from the spacer boundary. As a result, too strong of a STO current attenuates the AC field strength.